"I've always wanted to come over there and I think this is a good time. I didn't want to pass up the chance to join Hull KR and I can't wait to be part of such a great club," 23-year-old Kelly told the club's official website.
"I'm really excited to come over and play a new brand of football in Super League. I enjoy the unstructured side of the game that gets the fans on their feet and I think Super League will suit the way I like to play.
"If we can both stay together in the team we're confident it will be a really good year for Hull KR. I can't wait to get the show started, it's a new side and I'm looking forward to coming over, training hard and getting the combinations working."
Blair, 30, has previously played for Melbourne Storm and the Penrith Panthers.